01 Galant - antifreeze low, but no leaks?

I have an 01 Galant with 120k miles and the 4 cylinder engine. I recently had it in the shop for an oil change and inspection, and they told me my coolant was gone, none in reservoir and none in radiator. They filled it back up, but 2 days later, the coolant in the reservoir was nearly gone again. The car runs a little hotter than it used to (about 1/2 way up the temp gauge), but doesn't overheat.

I had it pressure tested, and it passed that with no leaks at all. They also ran a block test on it to check for internal damage, but that also showed no signs. They attributed the low antifreeze to air bubbles that had built in the lines due to no antifreeze being in there, that once they cleared, my antifreeze levels were low compared to when the air bubbles made them look artifically full. They refilled the reservoir.

I looked again this morning (engine cold), and the reservoir is a little below the "LOW" point again. However, after the car warms, or if I check it after driving with the car idling, the reservoir level is in between the "LOW" and "FULL" marks.

Any suggestions? Is there really a problem, given that I passed the 2 tests? If not, why is it now running a little hotter than it used to? I have not noticed any leaks on the ground, and don't really smell anything either. Any help is appreciated!!